This invention relates to a method for prolonging the useful life of filaments used in electron microbeam tools. In particular, this invention relates to a method of controlling the power to the filament to the saturation point and then controlling the power reduction when the apparatus is turned off.
Electron microbeam tools utilize filaments which are typically lanthanum hexaboride (LaB.sub.6). The filament may be a single crystal or polycrystalline material. Such filaments are used in devices such as scanning electron beam microscopes and the like. The filament is typically mounted on carbon filament ribbons and heated by current supplied to the ribbons. In operation such filaments typically operate at approximately 6 watts of DC power. In order to be acceptable processing and diagnostic tools, these filaments must be stable, that is, precisely immobile, with no fluctuations in current during operation. The lifetime of such a filament is critical to continuous operation of the instrument and to reduce the down time of the device. Each time the filament is replaced, the instrument must be realigned and pumped down, both time consuming procedures.
Stability of the filament current is also crucial--any change will disturb the filament resulting in warpage thereby shortening its lifetime. Such warpage also affects system resolution which, in the case of an E-beam, is in the Angstrom range. A second deficiency related to changes in filament current is contamination, that is, a deposition of a contaminant on the filament tip. Contamination reduces the emission of filament electrons.
With prior art the filament power is generally manually increased at various rates up to the saturation point, that is, the point at which an increase in current filament does not result in a further increase in filament power. There is, however, no regime or program for increasing filament power to the saturation point and then correspondingly decreasing filament power once operation is complete. Thus, within the prior art the usable lifetime of filaments is generally 100-200 hours.
U.S. Pat. No. 2,786,949 relates to a technique for controlling the voltage to X-ray tube devices. In particular, in this patent, a system is disclosed for the partial automation and the prevention of power restoration following interruption. A technique of X-ray tube warm-up is disclosed wherein the power is turned up at a gradual rate, usually within 30 seconds to one minute, as opposed to an abrupt application of full power. Thus, tube warm-up is not disclosed relative to a program gradual power increase at a specific and controlled rate. Moreover, this prior art device does not recognize that X-ray tubes are subjected to severly short lifetime problems as in the case of electron beam filaments. That is, in this patent, the high voltage of the X-ray tube, and thus the cathode current rather than the filament current is controlled. Given the heat generation in these devices, water is generally supplied for purposes of cooling and therefore the lifetime of the tube would not be appreciably improved since the cathode current, and not the bombardment, potential governs the X-ray tube lifetime. Consequently, turning up the power by the technician is a technique of controlling the high voltage to the tube.
Other techniques for preventing damage of electronic devices by controlling the power are set forth in the prior art. Reference is made to U.S. Pat. No. 3,810,025 relating to a technique of protecting the emitter of an electron gun by decreasing the impedance between the emitter and its associated electrode when an electrical breakdown occurs. U.S. Pat. No. 4,112,334 discloses and ignition circuit for extending the life of a gas-filled electrical lamp. The circuit applies a first level of power to an electrode prior to ignition followed by increasing the level of power within a proscribed time frame.